Shell catalysts and methods for their production are known in the state of the art. Particular shell catalysts are produced for example by applying a washcoat suspension containing a powdery support oxide and a catalytically active species to a support structure in the form of a shell. With shell catalysts, a more selective reaction control is possible in many cases than with catalysts which are loaded with the catalytically active species into the core of the support structure.
Phthalic anhydride (PA) for example is currently produced predominantly by means of shell catalysts in high selectivity. The great majority of the shell catalysts currently used for producing PA are shell catalysts with a shell comprising V2O5/TiO2 on a non-porous steatite support formed as a hollow cylinder. The active-metal oxide V2O5 is probably not present in the V2O5/TiO2 system of these catalysts only in the form of crystalline V2O5 particles. Such shell catalysts are usually produced by coating using appropriate suspensions in coating drums or fluid-bed units.
However, the above-named techniques have reached their limits as regards the homogeneity of the layer thickness of correspondingly produced shell catalysts, in fact as regards both the homogeneity of the layer thickness of an individual shell catalyst and that of a batch of shell catalysts.